Phase transformation during the heat treatment of sol-gel mullite

The mechanism of mullite formation depends upon the method of combining the silica and alumina-containing reactants. In this work, the effects of the processing variables (dilution, pH, gelling temperature and water content) on mullite formation were investigated. Monophasic gel was synthesized from aluminium nitrate nanohydrate dissolved in an absolute ethyl alcohol and silica sol mixture. The gelling was performed at room temperature and at 60 degrees C. It was found experimentally that the temperature of mullite formation depends on the microstructure of the gel. The lowest temperature of mullite formation (970 degrees C) was observed when the mullite gel was prepared at low pH (pH=1), low water content and high gelling temperature.